1. Field of the Invention
The present invention relates to a radar apparatus which radiates electromagnetic waves having a main lobe and grating lobes from an array antenna, receives the electromagnetic waves containing the main lobe reflected from a target in another array antenna, and produces a detection signal from the received electromagnetic waves, while suppressing influence of the grating lobes of the radiated electromagnetic waves on the detection signal, to detect the target located in a detection area.
2. Description of Related Art
An array antenna generally radiates electromagnetic waves having a main lobe and grating lobes, and another array antenna receives these electromagnetic waves. An antenna system having these antennas detects a target, reflecting the main lobe of the radiated electromagnetic waves in a detection area, from the received electromagnetic waves. In this case, to precisely detect the distance between the system and the target and the bearing angle from the system to the target, it is required to suppress the influence of the grating lobes on the received electromagnetic waves.
Japanese Patent Specification No. 4147447 discloses an array antenna system for suppressing influence of grating lobes. In this technique, this system contains a transmission array antenna constituted of a plurality of antenna elements on the grating suppressed side, a transmission processing section connected with this transmission array antenna, a reception array antenna constituted of a plurality of antenna elements on the grating suppressing side, and a reception processing section connected with this reception array antenna. The transmission array antenna radiates electromagnetic waves having a main lobe and grating lobes in a radiation directivity, and the reception array antenna receives electromagnetic waves in a reception directivity. The radiation directivity on the grating suppresses side is expressed by a power pattern of the radiated electromagnetic waves with respect to the direction of the radiated electromagnetic waves. The reception directivity on the grating suppressing side is expressed by a pattern of sensitivity to the electromagnetic waves incident on the reception array antenna with respect to the direction of the electromagnetic waves.
The reception directivity is set to have a single null point in a null point direction corresponding to the radiation direction of each grating lobe. Therefore, the sensitivity of the reception array antenna to electromagnetic waves coming from directions of a null point area including the null point direction of each null point is extremely lowered. Further, as compared with the power of the radiated main lobe, the power of each radiated grating lobe is sufficiently high in a directional area of the grating lobe.
With this structure of the system, the processing sections control the array antennas such that the direction of each grating lobe radiated from the transmission array antenna always coincides with the direction of one null point of the reception directivity. When the transmission array antenna radiates electromagnetic waves having a main lobe and grating lobes such that the main lobe is radiated in a detection area, a target placed in the detection area reflects the main lobe of the electromagnetic waves, the reception array antenna receives the electromagnetic waves containing the main lobe, and the reception processing section detects the target from the received electromagnetic waves. Because the direction of each grating lobe on the grating suppressed side coincides with the direction of one null point set in the reception directivity on the grating suppressing side, when the grating lobe width of the grating lobe area on the grating suppressed side is equal to or narrower than the null width of the null point area on the grating suppressing side, the grating lobe reflected from an object is not contained in the received electromagnetic waves. Therefore, when the grating lobe width of the grating lobe area is equal to or narrower than the null width of the null point area, the power of each whole grating lobe can be suppressed in the received electromagnetic waves.
Therefore, when the grating lobe width of the grating lobe area is equal to or narrower than the null width of the null point area, the antenna system can substantially lower the sensitivity of the reception array antenna in the radiation directions of the grating lobes, and the antenna system can detect the target placed in the detection area.
However, when the grating lobe width of the grating lobe on the grating suppressed side is wider than the null width of the null point area on the grating suppressing side, the power of only a portion of each grating lobe is suppressed in the received electromagnetic waves, but the power of the whole grating lobe cannot be suppressed in the received electromagnetic waves. Therefore, it is difficult or impossible to sufficiently lower the sensitivity in the radiation directions of the grating lobes, and the antenna system cannot detect the target placed in the detection area with high precision.
Further, a snap-shot technique is well known. In this technique, each time the reception array antenna receives the electromagnetic waves in a scanning period of time, a reception signal is produced from the received electromagnetic waves. Then, the reception signals produced in a predetermined period of time are summed up to obtain a summed reception signal. In this summed reception signal, the signal level corresponding to the main lobe is increased as compared with the signal level of one reception signal. However, the signal level corresponding to each grating lobe is also increased in the summed reception signal. As a result, even when the snap-shot technique is used to detect the target, the precision in the detection of the target is not improved.